This invention relates to stoves, such as wood stoves, and in particular to a convergent flow stove in which combustion occurs only in a defined area at the bottom of the stove with the combustion and products of combustion being directed downwardly through an outflow passage.
Wood-burning stoves are well-known, having often, in the past, been a main, if not only source of heat in homes. With the advent of modern gas and oil-dried boilers and furnaces, wood stoves became less common, until recent years when dramatically increasing fuel costs led to a resurgence of demand for wood stoves which consume fuel such as wood which is abundant, renewable and often cheaper than the gas or oil it replaces or supplements.
A typical wood stove has a vertical combustion chamber, with an outlet flue at the top of the combustion chamber and a combustion air inlet at the bottom of the combustion chamber. When the combustion chamber is charged with wood and combustion initiated, combustion occurs in the normal upward direction throughout the entire charge of wood. Characteristically, operation of such stoves is smokey and the heating output is unsteady.
Recognizing the unsatisfactory operating characteristics of the typical wood stove many efforts in the past have been made to direct combustion gases downwardly through the zone of incandescent coals to promote combustion of gases and lower the temperatures of the fuel above the incandescent zone. These efforts have been moderately successful in achieving steady, smokeless operation. Typically, however, steady firing has been hindered by the accumulation of ash in the incandescent zone, which causes firing to drop off. A grate is usually provided so that the ash can periodically be "shaken out" to revive the fire. In addition, "down draft" stoves have been just as smokey as typical stoves at low heating outputs. The probable reason is that because of the reduced volume of incandescent coals at low heating outputs the coals are too dispersed on the grate to effectively ignite the gases passing through.
One example of the prior art, U.S. Pat. No. 4,194,487, discloses an apparatus that directs combustion downwardly, but which would not have a steady heating output because of ash accumulation, nor good combustion at low heating outputs because the reduced volume of coals is divided into two separate parts not mutually supportive in maintaining ignition temperatures for the gases passing through.
U.S. Pat. No. 4,102,318 discloses another down draft apparatus that would not promote complete combustion at low heating outputs since the grate bars prevent the reduced volume of coals from compacting at the outlet in sufficient density to assure temperatures necessary for ignition of the gases. It should also be noted that at low heating outputs, depending on the width of the clearance between the separator plate 19 and the side walls, it is probable that all the flow of combustion gases is upwards, as in a typical stove.
U.S. Pat. No. 253,144 discloses a stove for coal burning that has a vertical cylindrical configuration. Although a grate is not used, and no net accumulation of ashes would be expected in the combustion zone, it is probable that steady, smokeless combustion could not be achieved by firing wood logs. Because of the central metering and air introduction shaft, the logs would need to be inserted on end into the annular fuel chamber. Logs on end tend to burn to a point, presenting less surface area, with the consequence that firing drops off. Also, logs do not slide past each other, with the consequence that the slowest burning log (usually the one with the thickest cross section) could hold other logs up, again causing firing to drop off. Because the central metering and air introduction shaft is employed, much of the useful space in the fuel chamber is occupied, and the downward migration of the fuel is constricted with the result that fuel can also bridge within the fuel chamber without entering the combustion area. In this case also, the reduced volume of coals at low heating outputs is separated by the flared bottom of the central shaft and dispersed radially making it probable that sufficient temperatures could not be maintained to assure combustion of the gases.